x8664ISA

x8664ISA, commonly known as x86-64 or AMD64, is the 64-bit extension of the x86 instruction set architecture. It preserves backward compatibility with legacy 32-bit x86 software while adding 64-bit general-purpose registers, a larger address space, and new operating modes. The architecture was developed by AMD and first implemented in mainstream CPUs in the early 2000s; Intel later adopted it as Intel 64 (also known as EM64T).

Key design features include 16 general-purpose 64-bit registers (RAX–R15), an expanded vector register set (XMM0–XMM15), and a 64-bit instruction pointer (RIP). The architecture provides a 64-bit mode for native 64-bit software and a compatibility mode to run 32-bit code. Memory addressing uses a theoretical 2^64 address space, but current processors implement a 48-bit canonical virtual address space, with physical addressing commonly up to 52 bits on many CPUs. Long mode uses a four-level paging structure (PML4, PDPT, PD, and PT) for address translation. Canonical addressing requires the upper bits to be sign-extended from bit 47.

Virtualization and extensions are integral to x8664ISA. Modern processors include virtualization features (Intel VT-x and AMD-V) and ongoing instruction-set enhancements such as AVX, AVX2, and AVX-512, which broaden performance for scientific computing, multimedia, and data-intensive workloads.

Software and ecosystem support is widespread. Major operating systems including Windows, Linux, and macOS run on x86-64, and compilers provide robust 64-bit ABIs and libraries. As the dominant 64-bit x86 architecture, x8664ISA underpins contemporary desktops, servers, and cloud computing, with continual refinement to support larger address spaces and higher parallelism.